Production of calcium chromate



Patented Jan. 30, 1945 UNITED .STATES- PATENT oi-"r cs PRODUCTION OF CALCIUM CHBOMATE Evan A. MacColl, Sault Ste. Marie, Ontario. Canada, assignor to Chromium Mininl' and Smelt--v in; Corpm, Limited, Sault Ste. Marie, Ontario, Canada, a corporation of Canada No Drawing. Application April 2, 1942, Serial No. 437,363

(01. 23-56) v trated solutions may be employed most effectively. I

2 Claims.

This invention relates to chromium recovery and has for an object the provision of an improved method or process for recovering chromium in the form of a valuable commercial comp und of chromium from crude chromium-liearing material such, for example, as chromite ore and high-carbon ferrochrom'ium. More particularly, the invention contemplates the provision of an improved method or processfor recovering chromium in the form of relatively pure calcium chromate from crude chromium-bearing material. The invention further contemplates the provision of an improved method or process for Producing calcium chromate. 1

Calcium chromate is a desirable reagent for use in many industries, but it is not widely used Mixing may be effected atthe maximum temperatures of, precipitation or at lower temperatures.

As indicated above, the heat treatment results in the production of an aqueous solution of sodium nitrate and a solid precipitate consisting essentially of calcium chromate. The calcium chromate precipitate and the solution of sodium because its cost of production when produced according to heretofore customary methods or processes is excessive. The present invention provides economical and emcient method of producing calcium chromate.

The invention is based on my discovery that calcium chromate may beproduced readily by reacting sodium chromate with calcium nitrate; I have found. that calcium nitrate and sodium chromate in water solution react rapidly'and completely to produce solid calcium chromate and an aqueous solution of sodium nitrate when the solution'is heated. Calcium chromate is formed and precipitated readily when a concen trated solution containing'sodium chromate and calcium nitrate is heated to a boiling temperature under atmospheric pressure. The precipitate formed when the solution is heated or boiled under atmospheric-pressure mayv comprise one or more hydrated forms of calcium chromate. An-.

hydrous calcium chromate may be formed by ature in excess of about 180 0.. (equivalent to a pressure of about 150 pounds per square inch).

- In a preferred method or process of the invention, the calcium nitrate and sodium chromate are dissolved separately in waterto form separate solutions of calcium nitrate and sodium chromate, andthe two solutions are mixed to form an aqueous solution containing both calcium nitrate and sodium chromate. The solution containing sodium chromate and calcium nitrate is then heated to efiect precipitation of calcium .chromate. I

Solutions of sodium chromate and. calciumnitrate may be separated by filtering or by sedi. mentation and decantation. The calcium chromate precipitate may be freed of adhering liquid I as such or in the form of limestone (08.00:), a

chromium-bearing material such as chromite ore or high-carbon ferrochromium and sodium nitrate (ora decomposition product of sodium ni-.

trate containing the sodium oxide of the sodiumheating the solution under pressure to a tempernitrate). The components of the charge preferably are in the form of particles sufficiently small to pass a 100-mesh screen, and oxidation preferably is carried out at a temperature in the range 1000 C. to 1200 C. The sodiumoxidebearing material preferably is employed in amount sufllcient to form sodium chromate with all of the chromium present in'the charge. Lime is employed as a diluent in amount suflicient to maintain the charge sufliciently porous to permit eifective oxidation. The oxidation or roasting treatment results in oxidation of the chromium i of the charge from the elemental state or the so one solution oi sodium chromate.

nitrate of any suitable concentrations maybe a employed. Precipitation of calcium chromate is more eifective and more complete when concentrated solutions are employed. Highly concentrivalent state with the production of chromium trioxide which combines with the sodium oxide to form sodium chromate. The oxidizedproduct of the roasting treatment is subjected to a leaching treatment with an aqueous liquid in accordance with known procedures to produce an aque-,

Sodium chromate solution thus obtained is mixed, after concentration it necessary, with a solution of calcium nitrate, .and the resulting mixture is heated to eflect the precipitation of calcium chromate. The mixed solution preterably is heated under pressure to a temperature in excess of about 180 C. toefiect the precipitation of anhydrous calcium chromate and the production 01' a solution of sodium nitrate.

The precipitated calcium chromate is separated from the sodium nitrate solution and it may be washed or dried or both in any suitable manner.

The sodium nitrate produced preferably is uti: lized in the oxidation treatment of additional crude chromium-bearing material to produce additional sodium chromate. The sodium nitrate may be utilized (1) by wetting an oxidation charge material with the solution, (2) by evapcrating the solution to dryness and incorporat ing the resulting solid sodium nitrate in an oxidation charge, or (3) by treating the sodium nitrate to recover its oxide oi nitrogen content and incorporating the residual sodium oxide content in an oxidation charge.

I claim:

1. The method of producing anhydrous calcium chromate which comprises forming an aqueous solution comprising calcium nitrate and sodium chromate, and heating the resulting solution under pressure to a temperature higher than I I about 180 C. and under conditions ensuring the- .crystaliization of anhydrous calcium chromate.

2. The method of producing anhydrous calcium' chromate which comprises subjecting a charge comprising chromium-bearing material, lime and sodium oxide-bearing material to an oxidizing treatment to form a product containing sodium ,chromate, subjecting the product of the oxidizing treatment to a leaching treatment and forming an aqueous solution of sodium chromate, subjecting the solution of sodium chromate to the action of calcium nitrate at an elevated temperature above about 180 C, under ouper-atmospheric pressure and under conditions ensuring the crystallization of anhydrous calciurn chromate to form solid calcium chromate and an aqueous solution of sodium nitrate, sepsrating the sodium nitrate and the calcium chr0- mate, and utilizing the sodium oxide content of the sodium nitrate in an oxidizing treatment oi chromium-bearing material to produce additional sodium chromate.

EVAN A. MACCOL'L. 

